Which is the best way to model a low poly coat?
For low poly clothes you could start by blocking out the shape with just vertices and edges. Begin with a simple plane delete two vertices, add a mirror modifier and start extruding vertices around your model with E and connecting vertices with F.



Then when you get something like in the picture, you can start connecting those edges and pairs of vertices to make faces around your model using F in edit mode.



When you get a simple geometry that works you can add loops to refine the model with Ctrl+R. Then you can add a solidify modifier and play with the thickness parameter to give volume to the cloth.

For the details as it's a low poly model you'd have to texture them, but you can simplify the process by sculpting first then baking the results and applying a normal map or displacement map to get volume.